Selective adsorption of gaseous alkenes into non-porous copper(I) complexes: controlling heat of adsorption and loading pressure

ABSTRACT

Disclosed are air-stable small-molecule adsorbents trimeric [Cu—Br] 3  and [Cu—H] 3  that undergo a reversible solid-state molecular rearrangements to [Cu—Br.(alkene)] 2  and [Cu—H.(alkene)] 2  dimers. The reversible solid-state rearrangement allows one to break adsorbent design trade-offs and achieve low heat of adsorption while retaining high selectivity and uptake.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Provisional Application 63/042,884, filed Jun. 23, 2020, which is incorporated by reference herein in its entirety.

STATEMENT ACKNOWLEDGING FINANCIAL SUPPORT

This invention was funded in part by The Welch Foundation under grant number Y-1289.

This invention was made with government support under grant no. CHE 1954456 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

Light alkenes such as ethene and propene are produced by cracking hydrocarbon feedstocks such as ethane, propane and naphtha (Froment, G., Chem. Eng. Sci., 1981, 36(8):1271-1282). The global demand and production of alkenes are higher than any other organic chemical. Annual global production of ethene and propene exceeds 200 million tonnes, and they are used to produce numerous products including polymers and olefin oxides (e.g., ethylene oxide) (Sholl, D. S. et al., Nature News, 2016, 532(7600):435).

Industrially, the separation of alkenes from unconverted alkanes is achieved by cryogenic distillation, which requires high pressures and low temperatures due to the similarities in their boiling points and volatility. For example, a distillation column with more than 100 trays operates at temperatures around −25° C. and pressures higher than 2000 kPa (Wu, Z., et al., Ind. & Eng. Chem. Res., 1997, 36(7):2749-2756). This energy-intensive separation process contributes to almost 75% of the total alkene production cost (Anson, A., et al., Chem. Eng. Sci., 2008, 63(16):4171-4175) and accounts for about 0.3% of global energy use. Several methods have been investigated to reduce the energy consumption and the cost of these separation processes. These include membrane (Hayashi, J.-I., et al., Ind. & Eng. Chem. Res., 1996, 35(11):4176-4181; Staudt-Bickel, C., et al., J. Membr. Sci., 2000, 170(2):205-214; Azhin, M., et al., J. Ind. Eng. Chem., 2008, 14(5):622-638; Bux, H., et al., J. Membrane Sci., 2011, 369(1-2):284-289; Tsou, D. T., et al., Ind. & Eng. Chem. Res., 1994, 33(12):3209-3216), adsorption (Gücüyener, C., et al., J. Am. Chem. Soc., 2010, 132(50):17704-17706; Gücüyener, C., et al., J. Mater. Chem., 2011, 21(45):18386-18397; Shi, M., et al., Chem. Eng. Sci., 2010, 65(11):3494-3498; Shi, M., et al., Chem. Eng. Sci., 2011, 66(12):2817-2822), or hybrid separation methods that combine distillation with membrane processes (Moganti, S., et al., J. Membr. Sci., 1994, 93(1):31-44).

Complexing agents such as silver and copper have been explored to improve adsorption and membrane separation processes (King, C. J., Separation processes based on reversible chemical complexation. 1987, Wiley: New York. p. 760-774). These metals reversibly interact with then-electrons of alkenes (Khan, N. A., et al., J. Hazardous Mater., 2017, 325:198-213; Dias, H. V. R., et al., Eur. J. Inorg. Chem., 2008, 509-522; Yang, R. T., et al., Ind. & Eng. Chem. Res., 1996, 35(4):1006-1011). The π-electrons provide a distinguishing feature to separate alkenes from alkanes, enhancing the selectivity and capacity of materials, leading to process designs with higher product purity, recovery, and through-put.

What are thus needed are new materials and methods for adsorption of alkenes. The compositions and methods disclosed herein addresses these and other needs.

SUMMARY

In accordance with the purposes of the disclosed devices, systems and methods as embodied and broadly described herein, the disclosed subject matter related to devices and systems, methods of making said devices and systems, and methods of using said devices and systems. More specifically, disclosed herein is a composition comprising an alkene and a compound having Formula I:

Also, disclosed are compositions comprising complexes having Formula II

In Formula I and II, each X is, independent of the other, chosen from H, CH₃, CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br and I; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I. In the disclosed compositions, the alkene (Alkene) can be ethene, propene, 1-butene, or 2-butene, or mixtures thereof. In other examples, an alkane such as ethane, propane, or butane or mixtures thereof can be present. The alkene can be part of a gas or liquid stream. Methods of separating an alkene from a mixture of the alkene and an alkane are also disclosed, the method comprising contacting the compound having Formula I with the mixture and forming the complex having Formula II. The alkene can then be recovered from the complex having Formula II by reducing the pressure or by raising the temperature or by using both pressure and temperature variations. Articles comprising the compounds having Formula I and/or complexes having Formula II and a substrate are also disclosed.

Additional advantages will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the aspects described below. The advantages described below will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive.

BRIEF DESCRIPTION OF FIGURES

The accompanying drawings, which are incorporated and constitute a part of this specification, illustrate several aspects of the disclosure, and together with the description, serve to explain the principles of the disclosure.

FIG. 1 shows the structures of trinuclear {[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) and {[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) and their chemistry with ethene and propene leading to dinuclear [Cu—Br.(C₂H₄)]₂, [Cu—H.(C₂H₄)]₂, [Cu—Br.(C₃H₆)]₂, and [Cu—H.(C₃H₆)]₂.

FIG. 2 shows molecular structures of [Cu—H.(C₂H₄)]₂ (left) and [Cu—H.(C₃H₆)]₂ (right, illustrating only the trans-propene conformation). The [Cu—Br.(C₂H₄)]₂ and [Cu—Br.(C₃H₆)]₂ analogs also have dinuclear structures.

FIG. 3 is a graph showing ethene adsorption (solid symbols) and desorption (hollow symbols) isotherms of [Cu—Br]₃ at 0, 20, and 40° C.

FIG. 4 , top, is a top view of the powder diffraction patterns for the [Cu—H]₃ ethene loading experiment. Black and grey arrows show calculated major peak positions for [Cu—H]₃ and [Cu—H.(C₂H₄)]₂, respectively. FIG. 4 , bottom, is a top view of the powder diffraction patterns for the [Cu—H.(C₂H₄)]₂ ethylene desorption experiment. Black and grey arrows show calculated major peak positions for [Cu—H]₃ and [Cu—H.(C₂H₄)]₂, respectively.

FIG. 5 is a graph showing propene adsorption (solid symbols) and desorption (hollow symbols) isotherms of [Cu—Br]₃ at 0, 20 and 40° C.

FIG. 6 is a graph showing ethene adsorption/desorption cycles for the [Cu—H]₃/[Cu—H.(C₂H₄)]₂ complex. The [Cu—H]₃ was treated with ethene at 600 kPa for 15 minutes between desorption cycles. The resulting [Cu—H.(C₂H₄)]₂ complex was then exposed to at 100 kPa regular atmosphere for various times 1, 5 and 10 minutes. 1st: after initial vacuum desorption; 2^(nd) and 6^(th) peaks: after 10 min atmosphere exposure; 3^(rd) and 4^(th) peaks: after 1 min; 5^(th) peak: after 5 min.

FIG. 7 shows the syntheses of dinuclear 1-butene complexes, {[3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}₂ ([Cu—H.(C₄H₈)]₂) and {[4-Br-3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}₂ ([Cu—Br.(C₄H₈)]₂) from 1-butene and {[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) and {[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) precursors. Products with both trans- and cis-oriented 1-butene groups are illustrated.

FIG. 8 contains molecular structures of {[3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}₂ ([Cu—H.(C₄H₈)]₂, left) and {[4-Br-3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}₂ ([Cu—Br.(C₄H₈)]₂, right), and view of the boat-shaped Cu₂N₄ core in [Cu—H.(C₄H₈)]₂.

FIG. 9 is a plot of absolute temperature (K) and calculated Gibbs free energy change (kJ·mol⁻¹) based on experimental enthalpy and entropy values.

FIG. 10 shows selected deformation densities from the NOCV-EDA analysis, accounting for the σ-donation (left) and π-backbonding (right) in the formation of ethene (a) and 1 butene (b) complexes. Charge flow from red to blue.

FIG. 11 shows the adsorption of 1-butene into [Cu—Br]₃ (top) and [Cu—H]₃ (bottom) under a feed pressure of 180 kPa. The “Butene_X” label represents different runs where X is the run number.

FIGS. 12 and 13 show the mass loss with increasing temperature during TGA analysis on [Cu—Br]₃ (FIG. 12 ) and [Cu—H]₃ (FIG. 13 ) after being loaded with each gas separately (ethene and propene). Both [Cu—Br]₃ and [Cu—H]₃ rapidly lose bound alkenes when exposed to atmosphere. For [Cu—Br]₃ it appears that all gases are desorbed by heating the complex. The complex started to lose ethene at approximately 49° C. and propene at around 43° C. However, calculations showed that only around 40% of the adsorbed amount of each gas was released during the TGA. This indicates that the rest of the adsorbed gas was already released upon exposure to the atmosphere during sample preparation. For [Cu—H]₃ it appears that less than 2% of ethene and propene were released during the TGA, which means that they were already desorbed when the adsorbent cell was open to atmosphere. TGA was performed on an Alphatech SDT Q600 TGA/DSC under an inert nitrogen atmosphere. Samples were heated from 20° C. to 800° C. at a rate of 10° C. min⁻¹ (Note: dashed lines indicate the expected mass loss.)

DETAILED DESCRIPTION

Solid adsorbents are of interest due to the potential efficiency of solid/gas operations such as temperature and pressure swing adsorption (Bao, Z., et al., Energy & Environ. Sci., 2016, 9(12):3612-3641). Adsorbent design currently requires trade-offs between desirable properties. Increasing capacity via surface area decreases selectivity; increasing selectivity via strengthening interactions also increases heat of adsorption and affects isotherm shape; and using absorption instead of adsorption decreases kinetics. The cost of these trade-offs is manifested in process design. For example, decreasing overall heat of adsorption is economically preferable because it means less cooling or heating energy is required to maintain the adsorbent temperature during operation.

Adsorbents with ‘step’-shaped isotherms, where the majority of gas uptake occurs over a narrow pressure range, could be applied to pressure or temperature swing processes requiring relatively small amounts of energy (McDonald, T. M., et al., Nature, 2015, 519(7543):303-308). Ideally, isotherm ‘steps’ would occur above atmospheric pressure at moderate temperatures (ca. 100 kPa, 25° C.) to avoid the capital and operational expense of vacuum swing adsorption. However, there are few known mechanisms for achieving ‘step’-shaped isotherms, limited to ‘gate-opening’ (Nijem, N., et al., J. Am. Chem. Soc., 2012, 134(37):15201-15204) and cooperative adsorption of carbon dioxide in metal organic frameworks (McDonald, T. M., et al., J. Am. Chem. Soc., 2012, 134(16):7056-7065; Siegelman, R. L., et al., J. Am. Chem. Soc., 2017, 139(30):10526-10538), small-molecule adsorbents (Cowan, M. G., et al., Angew. Chem., 2015, 127(19):5832-5835; Jayaratna, N. B., et al., Angew. Chem., 2018, 130(50):16680-16684), and hydride salts (Fossdal, A., et al., J. Alloys Comp., 2005, 397(1-2):135-139). Through the Claussius-Clapeyron relationship, positioning the ‘step’ pressure above 100 kPa at moderate temperature requires larger heat of adsorption, an undesirable trade-off due to the extra operational energy required from large heats of adsorption. Another trade-off is that step-shaped isotherms also produce non-sharpening wavefronts in breakthrough configurations, requiring alternate separation process designs to maximize adsorbent productivity.

In the present disclosure, traditional trade-offs for adsorbent design are avoided using an olefin-responsive, solid-state structural rearrangement mechanism. The air-stable and cheap trimeric complexes {[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) (Hettiarachchi, C. V., et al., Inorg. Chem., 2013, 52(23):13576-13583) and {[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) (Dias, H. V. R., et al., J. Fluorine Chem., 2000, 103(2):163-169) (FIG. 1 ) rearrange to the dimeric species [Cu—Br.(C₂H₄)]₂ and [Cu—H.(C₂H₄)]₂, respectively. As alkene adsorbent materials, they feature high capacity, high selectivity, fast rates of adsorption and desorption, and low heat of adsorption for the gaseous alkenes ethene and propene compared to other alkene adsorbents. Furthermore, [Cu—H]₃ undergoes alkene adsorption above 100 kPa at 20° C. and rapid desorption when exposed to atmosphere, allowing operation above atmospheric pressure and avoiding the requirement for vacuum swing adsorption process designs.

Compositions

In specific examples disclosed herein is a compound having Formula I:

wherein each X is, independent of the other, chosen from H, CH₃, CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br and I; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I. In specific examples, X can be chosen from H, Br, CF₃, and CH₃, e.g., H or Br.

In other specific examples, Y can be CF₃.

wherein each X is, independent of the other, chosen from H, CH₃, CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br and I.

In further examples, disclosed herein are compositions comprising an alkene and the compound having Formula I. The alkene can be ethene, propene, 1-butene, 2-butene or mixtures thereof. Further the composition can comprise an alkane. The alkanes can be ethane, propane, butane or mixtures thereof.

In still further examples, disclosed herein is a complex having Formula II:

wherein each X is, independent of the other, chosen from H, CH₃, CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br and I; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I. In further examples, X can be chosen from H, Br, CF₃, and CH₃, e.g., H or Br.

In other specific examples, Y can be CF₃.

wherein each X is, independent of the other, chosen from H, CH₃, CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br and I.

In still other examples, Alkene can be ethene, propene, 1-butene, 2-butene, or mixtures thereof. Without wishing to be bound by theory, the complexes of Formula II can be present with compounds of Formula I and an alkene, and such compositions are expressly contemplated and disclosed herein.

It should be noted that while Formula II shows a solid line between a copper atom and “Alkene”, this is not meant to imply a single (σ) bond. It is meant merely to illustrate a coordination of the copper to the alkene.

In still further examples, disclosed herein are adsorption materials comprising the composition of Formula I and/or Formula II and a substrate. The substrate can be a bead, film, particle, or membrane, which can be made of either an inorganic or polymeric substrate. In some examples, the adsorption materials can be in a fixed and/or fluidized bed, e.g., in a fixed and/or fluidized bed temperature and/or pressure swing adsorption process.

Methods

Disclosed herein are methods of separating an alkene from a mixture comprising the alkene and an alkane, comprising contacting the mixture with the compound having Formula I to form a complex having Formula II, wherein the Alkene moiety in the complex is the alkene being separated from the mixture. In some examples, the mixture can be contacted with the compound having Formula I at a pressure below a partial pressure of the Alkene, e.g., the alkene's partial pressure is above the pressure of the contacting step in the isotherm at the operating temperature. In specific examples, contacting the compound having Formula I with the mixture can occur at pressures at or above ambient pressure.

The alkene can be ethene, propene, 1-butene, 2-butene, or a mixture thereof. The alkane can be ethane, propane, butane or mixtures thereof. In some embodiments the alkane can be separated from other gas mixtures. While not wishing to be bound by theory, the alkene can be separated from any gas that does not contain a carbon-carbon double bond and/or pi electrons that would interact with the compounds having Formula I. For example, alkenes can be separated from N₂, methane, carbon dioxide.

The mixture can be contacted with the composition having Formula I at any temperature up to the decomposition temperature of the compounds having Formula I, which can be up to approximately 200° C. In some specific examples, the mixture can be contacted with the composition having Formula I at from 0° C. to 200° C., from 0° C. to 150° C., from 0° C. to 100° C., from 0° C. to 65° C., from 0° C. to 50° C., from 0° C. to 40° C., from 0° C. to 30° C., from 0° C. to 20° C., from 0° C. to 10° C., from 10° C. to 40° C., from 10° C. to 30° C., from 10° C. to 20° C., from 20° C. to 40° C., from 20° C. to 30° C., or from 30° C. to 40° C. In further examples, the mixture can be contacted with the composition having Formula I at 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200° C., where any of the stated values can form an upper or lower endpoint of a range.

In still other examples, the mixture can be contacted with the composition of Formula I at pressures from ambient pressure to 100 kPa. In still other examples, the mixture can be contacted with the composition of Formula I at pressures from 100 kPa to 100,000 kPa, e.g., from 600 kPa to 1000 kPa. In specific examples the pressure can be 100 kPa, 200 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa, 700 kPa, 800 kPa, 900 kPa, 1000 kPa, 2000 kPa, 5000 kPa, 10,000 kPa, 50,000 kPa, or 100,000 kPa, where any of the stated values can form an upper or lower endpoint of a range.

In other examples, the pressure can be reduced to ambient pressure or below after forming the complex having Formula II and the alkene can be isolated or recovered. In other examples, the temperature can be increased after forming the complex having Formula II and the alkene, can be isolated or recovered. Still further, the pressure and temperature can be adjusted to conditions that result in the release of the alkene from the complex having Formula II and the alkene can then be isolated or recovered.

In still further examples, the method can be a solid-state method wherein the compound having Formula I is in its solid state when contacted with the alkene. Yet in other examples, the compound having Formula I can be contacted with the alkene in the presence of a solvent. Examples of suitable solvents include methylene chloride and chloroform.

Examples

To further illustrate the principles of the present disclosure, the following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compositions, articles, and methods claimed herein are made and evaluated. They are intended to be purely exemplary of the invention and are not intended to limit the scope of what the inventors regard as their disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperatures, etc.); however, some errors and deviations should be accounted for. Unless indicated otherwise, temperature is ° C. or is at ambient temperature, and pressure is at or near atmospheric. There are numerous variations and combinations of process conditions that can be used to optimize product quality and performance. Only reasonable and routine experimentation will be required to optimize such process conditions.

All manipulations were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques unless otherwise noted. Solvents were purchased from commercial sources and distilled prior to use. NMR spectra were recorded at 25° C. on a JEOL Eclipse 500 (¹H, 500.16 MHz; ¹³C, 125.78 MHz; ¹⁹F, 470.62 MHz), unless otherwise noted. Proton and carbon chemical shifts are reported in ppm versus Me₄Si. ¹⁹F NMR values were referenced to external CFCl₃. Melting points were obtained on a Mel-Temp II apparatus and were not corrected. Raman data were collected on a Horiba Jobin Yvon LabRAM Aramin Raman spectrometer with a HeNe laser source of 633 nm. The {[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) (Hettiarachchi, C. V., et al., Trinuclear Copper (I) and Silver (I) Adducts of 4-Chloro-3,5-bis(trifluoromethyl) pyrazolate and 4-Bromo-3,5-bis (trifluoromethyl) pyrazolate. Inorg. Chem. 2013, 52(23):13576-13583) and {[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) (Dias, H. V. R., et al., Coinage metal complexes of 3,5-bis (trifluoromethyl) pyrazolate ligand: Synthesis and characterization of {[3,5-(CF₃)₂Pz]Cu}₃ and {[3,5-(CF₃)₂Pz]Ag}₃ . J. Fluorine Chem. 2000, 103(2):163-169) were prepared according to reported literature procedures with a slight modification. Gas sorption measurements were performed using a volumetric adsorption machine (Quantachrome-Autosorb-iQ2). In situ high-gas pressure diffraction data of [Cu—H]₃ in ethene atmosphere were collected using the monochromatic X-rays available at the 17-BM (0.45238 Å) beamline (300 μm diameter beam size) at the Advanced Photon Source, Argonne National Laboratory in combination with a VAREX 4343 amorphous-Si flat panel detector. Thermogravimetric analysis (TGA) was performed on an Alphatech SDT Q600 TGA/DSC under an inert nitrogen atmosphere. Samples were heated from 20° C. to 800° C. at a rate of 10° C.·min⁻¹. All other reactants and reagents were purchased from commercial sources.

Synthesis of [(4-Br-3,5-(CF₃)₂Pz)Cu(H₂C═CH₂)]₂, referred as ([Cu—Br.(C₂H₄)]₂)

{[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) (0.3 g, 0.289 mmol) was dissolved in ˜10 mL of dichloromethane, and a gentle stream of ethene was bubbled into the solution for ˜8-10 min. The solution was kept at −20° C. to obtain X-ray quality colorless crystals of [Cu—Br.(C₂H₄)]₂. Yield: 95%. M.p.: 210-215° C. (melted with a temperature similar to that observed for [Cu—Br]₃ indicating the clean loss of ethene). Raman (neat, cm⁻¹): 3097, 3081, 3062, 2992, 1909, 1542, 1517, 1443, 1361, 1281, 1188, 1179, 1158, 1140, 1035, 968, 960, 815, 746. Room temperature NMR data: ¹H NMR (in CDCl₃): δ (ppm) 4.64 (br s, 2H, CH₂). ¹⁹F NMR (in CDCl₃): δ (ppm) −60.07 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 85.4 (br s, CH₂═CH₂), 91.3 (s, C-4), 120.6 (q, ¹J_(C-F)=270.3 Hz, CF₃), 141.2 (q, ²J_(C-F)=35.6 Hz, C-3/C-5). [Cu—Br]₃ and free ethene generated due to ethene dissociation from [Cu—Br.(C₂H₄)]₂ are also present in the mixture. Low temperature (−60° C.) NMR data. ¹H NMR (in CDCl₃): δ (ppm) 4.45 (br s, 2H, CH₂). ¹⁹F NMR (in CDCl₃): δ (ppm) −59.67 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 84.0 (br s, CH₂═CH₂), 91.1 (s, C-4), 120.2 (q, ¹J_(C-F)=262.7 Hz, CF₃), 140.5 (q, ²J_(C-F)=37.6 Hz, C-3/C-5). The presence of traces of [Cu—Br]₃ generated as a result of ethene dissociation from [Cu—Br.(C₂H₄)]₂ was observed as very minor signals in the mixture.

Synthesis of [(4-Br-3,5-(CF₃)₂Pz)Cu(H₂C═CHCH₃)]₂ ([Cu—Br.(C₃H₆)]₂)

A dichloromethane solution of ([Cu—Br]₃) (0.25 g, 0.241 mmol) was concentrated by bubbling a gentle stream of propene gas through the solution and kept at −20° C. to obtain X-ray quality colorless crystals of [Cu—Br.(C₃H₆)]₂. Yield: 86%. M.p.:195° C. (melted at a temperature similar to that observed for [Cu—Br]₃). Raman (neat, cm⁻¹): 3080, 3002, 2977, 2928, 1546, 1517, 1447, 1356, 1264, 1164, 935, 894. Room temperature NMR data with excess propene: ¹H NMR (in CDCl₃): δ (ppm) 1.70 (br s, 3H, CH₃), 4.55 (br s, 2H, CH₂), 5.53 (br s, 1H, CH). ¹⁹F NMR (in CDCl₃): δ (ppm) −60.29 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 19.7 (s, CH₃), 83.4 (br s, ═CH₂), ═CHCH₃ peak could not be observed, 91.0 (s, C-4), 120.7 (q, ¹J_(C-F)=271.1 Hz, CF₃), 141.0 (q, ²J_(C-F)=38.4 Hz, C-3/C-5). Signals for free propene (present in excess) and [Cu—Br]₃ generated due to dissociation of propene from [Cu—Br.(C₃H₆)]₂ were also observed. Low temperature (−40° C.) NMR data with excess propene: ¹H NMR (in CDCl₃): δ (ppm) 1.62 (br s, 3H, CH₃), 4.20 (br s, 0.5H, CH₂), 4.26 (br s, 0.5H, CH₂), 4.37 (br s, 1H, CH₂), 5.25 (br s, 1H, CH). ¹⁹F NMR (in CDCl₃): δ (ppm) −59.84 to −60.20 (several singlets). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 19.8 (s, CH₃), 82.6 (br s, ═CH₂), 83.1 (br s, ═CH₂), 90.8 (s, C-4), 100.7 (br s, ═CHCH₃), 101.3 (br s, ═CHCH₃), 120.4 (q, ¹J_(C-F)=268.7 Hz, CF₃), 140.5 (q, ²J_(C-F)=36.4 Hz, C-3/C-5). No signs for the presence of [Cu—Br]₃ in the NMR spectra. Peaks for free propene (present in excess) were observed.

Synthesis of [(3,5-(CF₃)₂Pz)Cu(H₂C═CH₂)]₂ ([Cu—H.(C₂H₄)]₂)

{[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) (0.35 g, 0.437 mmol) was dissolved in ˜12 mL of dichloromethane and stirred for ˜10-12 min under a slow stream of ethene. The reaction mixture was concentrated with a continuous flow of ethene and kept at −20° C. to obtain X-ray quality colorless crystals of [3,5-(CF₃)₂Pz)Cu(H₂C═CH₂)]₂, ([Cu—H.(C₂H₄)]₂). Yield: 92%. M.p.: 180-185° C. (melted at a temperature similar to that observed for [Cu—H]₃ indicating the clean loss of ethene). Raman (neat, cm⁻¹), selected peaks: 2986, 1537, 1511, 1450, 1370, 1270, 1157, 1136, 997. Room temperature NMR data: ¹H NMR (in CDCl₃): δ (ppm) 6.84 (s, 2H, Pz-H), ethene signal appears as a broad peak at 5.08 ppm. ¹⁹F NMR (in CDCl₃): δ (ppm) −59.96 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 104.1 (s, C-4), 121.1 (q, ¹J_(C-F)=268.7 Hz, CF₃), 142.5 (q, ²J_(C-F)=36.8 Hz, C-3/C-5). [Cu—H]₃ and free ethene generated due to ethene dissociation from [Cu—H.(C₂H₄)]₂ are also present in the mixture. Low temperature (−60° C.) NMR data: ¹H NMR (in CDCl₃): δ (ppm) 6.85 (s, 2H, Pz-H), 4.48 (br s, 8H, bound CH₂═CH₂). ¹⁹F NMR (in CDCl₃): δ (ppm) −59.48 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 83.2 (s, bound CH₂═CH₂), 104.2 (s, C-4), 120.8 (q, ¹J_(C-F)=268.3 Hz, CF₃), 141.9 (q, ²J_(C-F)=37.2 Hz, C-3/C-5). Very minor amounts of free ethene and [Cu-11]3 generated due to ethene dissociation from [Cu—H.(C₂H₄)]₂ are also present in the mixture.

Synthesis of [(3,5-(CF₃)₂Pz)Cu(H₂C═CHCH₃)]₂ ([Cu—H.(C₃H₆)]₂)

{[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) (0.25 g, 0.312 mmol) was dissolved in ˜8 mL of dichloromethane and stirred for ˜8-10 min while bubbling propene as a gentle stream into the solution. The reaction mixture was concentrated with a continuous flow of propene and kept at −20° C. to obtain X-ray quality colorless crystals of [Cu—H.(C₃H₆)]₂. Yield: 91%. M.p.: 185° C. (melted at a temperature similar to that observed for [Cu—H]₃ indicating the clean loss of propene). Raman (neat, cm⁻¹): 3157, 2960, 2901, 1538, 1504, 1456, 1403, 1358, 1255, 1182, 1160, 1138, 988, 923, 888, 850, 800. Room temperature NMR data with excess propene: ¹H NMR (in CDCl₃): δ (ppm) 1.72 (br d, 6H, CH₃), 4.93 (br s, 2H, CH₂), 5.02 (br d, 2H, CH₂), 5.82 (br s, 2H, CH), 6.82 (s, 2H, Pz-H). ¹⁹F NMR (in CDCl₃): δ (ppm) −60.29 (s). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 103.6 (br, C-4), no other peaks for [Cu—H.(C₃H₆)]₂ were observed. Peaks for free propene (present in excess) and [Cu—H]₃ generated as a result of dissociation of propene from [Cu—H.(C₃H₆)]₂ were also observed. Low temperature (−60° C.) NMR data with excess propene: ¹H NMR (in CDCl₃): δ (ppm) 1.59 (br m, 6H, CH₃), 4.19 (br d, 1H, CH₂), 4.29 (br d, 1H, CH₂), 4.36 (br s, 2H, CH₂), 5.25 (br s, 2H, CH), 6.82 (br s, 2H, Pz-H). ¹⁹F NMR (in CDCl₃): δ (ppm) −59.74 to −60.06 (several singlets presumably due to isomers resulting of cis/trans propene orientation). ¹³C{¹H} NMR (in CDCl₃): δ (ppm) 19.9 (s, CH₃), 81.7 (br s, ═CH₂), 82.0 (br s, ═CH₂), 99.0 (br s, ═CHCH₃), 99.5 (br s, ═CHCH₃), 103.6 (s, C-4), 120.6 (q, ¹J_(C-F)=220.7 Hz, CF₃), 141.4 (br q, C-3/C-5). Peaks for free propene (present in excess) were also observed.

Characterization

Trinuclear copper(I) complexes [Cu—Br]₃ and [Cu—H]₃ adsorb ethene and propene in a reversible manner both in solution and in solid state (FIG. 1 ). [Cu—Br]₃ and [Cu—H]₃ in CH₂C₁₂ solutions react with ethene to produce [Cu—Br.(C₂H₄)]₂ and [Cu—H.(C₂H₄)]₂, respectively, which were isolated under an ethene atmosphere as colourless crystalline solids at −20 C. Bulk purity was established using Raman spectroscopy to verify the presence of metal bound alkenes.

The X-ray crystal structures at 100 K of the ethene complexes [Cu—Br.(C₂H₄)]₂ and [Cu—H.(C₂H₄)]₂ show that they are dinuclear species, in contrast to the trinuclear [Cu—Br]₃ and [Cu—H]₃. The dimers adopt boat-shaped Cu₂N₄ cores (FIG. 2 ). The analogous copper(I) propene complexes [Cu—Br.(C₃H₆)]₂ and [Cu—H.(C₃H₆)]₂, were synthesized via a similar route and are also dinuclear structures and form crystals with two propene moieties adopting cis and trans-conformation in the solid state (FIG. 2 ), which is consistent with low temperature NMR spectroscopic data of these samples. From searches in the Cambridge Structural Database, [Cu—Br.(C₃H₆)]₂ and [Cu—H.(C₃H₆)]₂ appear to be the first examples of structurally characterized copper(I)-propene complexes. The occurrence of this remarkable gas-induced trimer to dimer conversion in the solid-state, involving the breaking and formation of several bonds, is the origin of the very attractive gas adsorption properties described below.

In CDCl₃ solution, trinuclear precursors and dinuclear products are in fast equilibrium (FIG. 1 ) on the NMR time scale and can be driven toward the copper-alkene products upon lowering the temperature. Van′t Hoff analysis of the ¹H, ¹⁹F and ¹³C VT-NMR data provided the enthalpy change for this alkene uptake in solution as −22 and −35 kJ·mol⁻¹ per Cu-ethene interaction for the formation of [Cu—Br.(C₂H₄)]₂ and [Cu—H.(C₂H₄)]₂, respectively. The corresponding propene adduct formations are slightly more exothermic at −34 and −39 kJ·mol⁻¹ per Cu-propene interaction, respectively. In general, propene has higher heat of adsorption than ethene, and the observation here is probably a reflection of favourable interactions between the more electron rich propene with the Lewis acidic Cu(I) sites (although many other factors can complicate such a simple explanation considering the large structural reorganization during alkene coordination process).

Solid samples of these copper-alkene complexes lose alkene at room temperature upon removal from alkene atmosphere, with the ethene adducts showing greater propensity for the alkene loss under similar conditions.

Single-gas adsorption isotherms were measured up to 100 kPa and 20° C. to quantify the pressure-dependent ethene uptake by [Cu—Br]₃ and [Cu—H]₃ in the solid-state. While [Cu—Br]₃ showed an ethene uptake of 2.51 mol_(ethene)·mol_(complex) ⁻¹, [Cu—H]₃ showed almost zero uptake below 100 kPa. The step-shaped isotherm (similar to IUPAC Type V) for [Cu—Br]₃ showed that ≤80% loading capacity can be obtained in one ‘step’ by increasing pressure from 30 to 35 kPa (FIG. 3 ).

The negligible ethene uptake of [Cu—H]₃ at 100 kPa and 20° C. suggested that the ‘step’ in ethene uptake had been shifted to pressures above 101 kPa. Exposing [Cu—H]₃ to higher pressures (636-682 kPa) using a house-built apparatus resulted in ethene loadings of ≤2.5 mol_(ethene)·mol⁻¹ _(complex).

Equilibrium adsorption isotherms were collected for [Cu—Br]₃ at 0, 20 and 40° C. (FIG. 3 ) to examine the effect of temperature on the step pressure and determine the heat of adsorption as −20.7 kJ·mol⁻¹ _(Cu3); −10.0 kJ·mol⁻¹ _(Cu.C2H4) interaction. For comparison, the heat of adsorption of ethene for the heavily fluorinated [Cu—CF₃]₃ was −38 kJ·mol⁻¹ _(Cu3); −13.1 kJ·mol⁻¹ _(Cu.C2H4) interaction (Jayaratna, N. B., et al., Angew. Chem., 2018, 130(50):16680-16684). The Clausius-Clapeyron relation predicts that the ‘step’ pressures of ethene adsorption at given temperature should increase with increasing heat of adsorption. However, the step pressures vary in the order [Cu—CF₃]₃<[Cu—Br]₃<[Cu—H]₃; contrary to prediction. This can be explained by considering that the observed heat of adsorption is the combination of the exothermic heat of adsorption from ethene binding and the endothermic trimer to dimer phase change induced by ethene coordination. These results demonstrate that the design strategy of increasing the energy of ethene binding while counter-balancing with increasing the phase change energy breaks trade-offs in adsorbent design and can be used to position the ethene uptake ‘step’ pressure independent of overall heat of adsorption.

To definitively attribute the mechanism of ethene uptake to rapid conversions between trimeric [Cu—H]₃ and [Cu—Br]₃ and their corresponding dimeric ethene complexes, in-situ PXRD measurements were performed at 17-BM beamline at the Advanced Photon Source, Argonne National Laboratory (FIG. 4 ). Specifically, in situ high-gas pressure diffraction data from [Cu—H]₃ and [Cu—Br]₃ in ethene atmosphere were collected using the monochromatic X-rays available at the 17-BM. Beams (300 μm diameter beam size) with 0.45238 Å wavelength were used for [Cu—H]₃ samples and 0.24117 Å wavelength was used for [Cu—Br]₃ experiments at the Advanced Photon Source, Argonne National Laboratory in combination with a VAREX 4343 amorphous-Si flat panel detector. Samples of [Cu—H]₃ and [Cu—Br]₃ were loaded into 1.0 mm quartz capillaries with glass wool on either side. The capillary with sample was then loaded into the gas flow-cell, to perform in situ PXRD experiments. At one end the gas cell was connected to a two-way valve which allowed changing between a 1 atm helium flow and a high-pressure syringe pump (Teledyne ISCO 500D) which was filled with ethene gas.

A remarkable solid-state to solid-state transformation of trinuclear copper precursors to dimeric [Cu—H.(C₂H₄)]₂ and [Cu—Br.(C₂H₄)]₂ was observed under a high pressure ethene atmosphere (FIG. 5 ). The existence of all structures was confirmed by correlation to the simulated PXRD data generated from single-crystal structures.

Under ethene flow at 100 kPa, there is little formation of [Cu—H.(C₂H₄)]₂ after 10 min. When the pressure of ethene was raised to 10 bar (1000 kPa) >95% [Cu—H]₃ instantly transformed into [Cu—H.(C₂H₄)]₂ (FIG. 4 , top). The entire conversion process completed in under 15 min. The resulting product is stable under an ethene atmosphere, however [Cu—H.(C₂H₄)]₂ converts back to [Cu—H]₃ when placed in a flow of helium. Complete conversion to [Cu—H]₃ is achieved within an hour, with the majority of conversion occurring within the first 15 minutes (FIG. 4 , bottom).

Incredibly, the remarkable trimer to dimer solid-state observed for ethene could be extended to the larger alkene propene. The propene equilibrium adsorption isotherms for [Cu—Br]₃ and [Cu—H]₃ at 20° C. showed uptakes of 2.17 and ca. 0 mol_(propene)·mol_(complex) ⁻¹ of [Cu—Br]₃ and [Cu—H]₃, respectively (FIG. 5 ). 90% of propene loading could be obtained by swinging the pressure between 45 and 75 kPa. This confirmed that even the larger alkene was able to penetrate the dense crystalline material and induce the reversible trimer to dimer structural rearrangement in the solid state.

As with ethene, increasing the temperature increased the ‘step’ pressure. However, no uptake was observed at 40° C., indicating that the step pressure at 40° C. is likely above 100 kPa. [Cu—H]₃ therefore has potential for use in a temperature swing adsorption process around 100 kPa where minor temperature changes of ca. 20° C. could lead to the adsorption/release of most of the alkene gas.

[Cu—H]₃ was tested for propene uptake at ca. 519 kPa and showed loading of 2.2 mol_(propene)·mol_(complex) ⁻¹. As with ethene, this exceptional behaviour raises potential for [Cu—H]₃ to be used in temperature swing adsorption processes operating above 100 kPa.

High alkene:alkane selectivities were observed because alkanes cannot coordinate to the copper(I) centres. Both [Cu—Br]₃ and [Cu—H]₃ showed low uptake of the alkanes ethane and propane (<0.1 mol_(gas)·mol_(complex) ⁻¹), as expected for solids with low surface area. Ideal ethene/ethane and propene/propane selectivities for [Cu—Br]₃ were calculated from the equilibrium loadings at 101 kPa and 20° C. as 47:1 and 40:1, respectively. The ethene/ethane selectivity of [Cu—H]₃ at 620 kPa was 47:1 and propene/propane selectivity at 440 kPa was 29:1. This selectivity is higher than most of the adsorbents reported in the literature.

In the past, investigations into small-molecule absorbents have suggested that the small surface areas will limit the gas adsorption and desorption rates. The surface areas of [Cu—Br]₃ and [Cu—H]₃ are less than 16 m²·g⁻¹ and uptake rates at 100 kPa are slow. Notably, the ethene and propene adsorption rates are fast (90% loading and >75% loading within 3 minutes) when [Cu—Br]₃ and [Cu—H]₃ are loaded at high pressure (>600 kPa and >440 kPa, respectively). Repeated cycles clearly demonstrate the reproducibility of these rapid adsorption and desorption rates.

Cycling a sample of [Cu—H]₃ between an ethene pressure of ca. 600 kPa and atmospheric pressure (i.e., opening the adsorption cell and exposing the sample to atmosphere) revealed that 90% of the adsorption capacity was regenerated within ≤10 minutes (FIG. 6 ). Likewise, propene is desorbed within <15 minutes when exposed to atmosphere. This relatively rapid rate of desorption is in contrast to the small-molecule adsorbents reported previously, which required extended periods under vacuum and elevated temperature to induce ethene desorption.

To probe the transport of ethene within these materials, Positron Annihilation Spectroscopy (PALS) was used to investigate changes in free-volume between the trimer and dimer configurations. The average free volume element sizes within [Cu—Br]₃ and [Cu—H]₃ were 0.581, and 0.301 nm; compared to the kinetic diameter of ethene (0.4163 nm) (He, Y., et al., Chem. Commun. 2012. 48(97):11813-11831). However, these internal free volumes did not correlate to the adsorption kinetics, with [Cu—H]₃ being faster. [Cu—H]₃ has the shortest lifetime, and hence smallest free volume element size, however it featured the highest intensity, therefore showing considerable free volume. The smaller free volume element size, 0.3 nm, is too small for the adsorption of ethene through the solid, potentially explaining why increased pressure is needed for the ethene to convert the structure.

The trimeric copper complexes [Cu—Br]₃ and [Cu—H]₃ undergo a remarkable solid-state transformation to dimeric species upon exposure to the gaseous alkenes ethene and propene, mimicking solution chemistry. This allows one to break trade-offs in adsorbent design between heat of adsorption and selectivity, isotherm ‘step’ pressure, and capacity. For [Cu—H]₃, adsorption is rapid at high pressures, and the reversibility was observed to occur within minutes indirectly (e.g., regeneration of adsorption capacity) after exposure to atmosphere, and directly via reduction of ethene partial pressure using helium gas (in-situ PXRD). Alkene capacity approaches 1 mol/mol loadings per copper site, and the low surface area of [Cu—Br]₃ and [Cu—H]₃ combined with the selectivity of the adsorption mechanism lead to high alkene/alkane ‘adsorption’ selectivities of >47 for ethene:ethane and >29 for propene:propane. Finally, the structural arrangement concurrent with the alkene adsorption results in low overall heat of adsorption 10-20 kJ mol⁻¹ _(ethene). In summary, the material [Cu—H]₃ embodies an ideal alkene/alkane adsorbent, breaking traditional trade-offs to achieve high capacity, selectivity, fast adsorption and desorption kinetics, low heat of adsorption, stability in ambient air, and process operation above atmospheric pressure.

Synthesis of {[3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}₂ ([Cu—H.(C₄H₈)]₂)

{[3,5-(CF₃)₂Pz]Cu}₃ ([Cu—H]₃) (0.4 g, 0.5 mmol) was dissolved in ˜10 mL of dichloromethane. The solution was concentrated with 1-butene and kept at −20° C. to obtain X-ray quality colorless crystals of [Cu—H.(C₄H₈)]₂. Yield: 86%. M.p.: 175-180° C. (melted at a temperature similar to that observed for [Cu—H]₃ indicating the clean loss of 1-butene). Raman (neat, cm⁻¹): 3161, 2973, 2933, 2903, 2870, 1534, 1504, 1443, 1245, 1135, 987, 931. Elemental data of the vacuum dried materials indicate the loss of butene and the formation of [Cu—H]₃. Room temperature NMR data: ¹H NMR (in CDCl₃): δ (ppm) 1.00 (t, J=7.3 Hz, 6H, CH₃), 2.06 (br s, 4H, CH₂), 4.28 (br s, 2H, CH₂), 4.42 (br s, 2H, CH₂), 5.39 (br s, 2H, CH), 6.81 (s, 2H, Pz-H). ¹⁹F NMR (CDCl₃): δ (ppm) −60.11 (s). ¹³C{¹H} NMR (CDCl₃): δ (ppm) 13.3 (s, CH₃), 26.9 (s, CH₂), 103.8 (s, C-4), 122.3 (CF₃), 142.2 (q, ²J_(C-F)=25.2 Hz, C-3/C-5). Peaks for [Cu—H]₃ and free 1-butene, that are in dynamic equilibrium with [Cu—H.(C₄H₈)]₂ were also observed. Room temperature NMR data with excess 1-butene: ¹H NMR (in CDCl₃): δ (ppm) 1.01 (t, J=7.3 Hz, 6H, CH₃), 2.06 (br s, 4H, CH₂), 4.92 (br s, 2H, CH₂), 5.00 (br s, 2H, CH₂), 5.87 (br s, 2H, CH), 6.82 (s, 2H, Pz-H). ¹⁹F NMR (CDCl₃): δ (ppm) −60.10 (s). ¹³C{¹H} NMR (CDCl₃): δ (ppm) 13.3 (s, CH₃), 26.9 (s, CH₂), 103.8 (s, C-4), 121.2 (q, ¹J_(C-F)=268.7 Hz, CF₃), 142.2 (q, ²J_(C-F)=37.6 Hz, C-3/C-5). Peaks for [Cu—H]₃ resulting from 1-butene dissociation and excess 1-butene were observed. These species are in dynamic equilibrium with [Cu—H.(C₄H₈)]₂. Low temperature (−40° C.) NMR data with excess 1-butene: ¹H NMR (in CDCl₃): δ (ppm) 0.98 (t, J=7.4 Hz, 6H, CH₃), 1.93 (br s, 4H, CH₂), 4.18 (br d, 1H, CH₂) 4.27 (br d, 1H, CH₂), 4.37 (br d, 2H, CH₂), 5.32 (br s, 2H, CH), 6.82 (s, 2H, Pz-H). ¹⁹F NMR (CDCl₃): δ (ppm) −59.61 to −59.97 (several fine singlets). ¹³C{¹H} NMR (CDCl₃): δ (ppm) 13.9 (s, CH₃), 14.4 (s, CH₃), 26.8 (s, CH₂), 79.4 (s, ═CH₂), 80.0 (s, ═CH₂), 103.7 (s, C-4), 106.3 (br s, ═CHC₂H₅), 106.6 (br s, ═CHC₂H₅), 120.9 (q, CF₃), 141.7 (q, ²J_(C-F)=36.0 Hz, C-3/C-5). No [Cu—H]₃ signals were observed at −40° C. Peaks for excess 1-butene were also observed.

Synthesis of {[4-Br-3,5-(CF₃)₂Pz]Cu(H₂C═CHC₂H₅)}2 ([Cu—Br.(C₄H₈)]₂)

{[4-Br-3,5-(CF₃)₂Pz]Cu}₃ ([Cu—Br]₃) (0.4 g, 0.386 mmol) was dissolved in ˜10-12 mL of dichloromethane and stirred for about 8-10 min while bubbling 1-butene. The reaction mixture was concentrated with a continuous flow of 1-butene and kept at −20° C. to obtain X-ray quality colorless crystals of [Cu—Br.(C₄H₈)]₂. Yield: 93%. M.p.: 180-185° C. (melted at a temperature similar to that observed for [Cu—Br]₃ indicating the clean loss of 1-butene). Raman (neat, cm⁻¹): 3074, 3013, 2982, 2936, 2903, 2878, 2846, 1535, 1506, 1434, 1341, 1272, 1250, 1166, 1133, 1023, 1007, 961, 930, 837, 821. Elemental data of the vacuum dried materials indicate the loss of butene and the formation of [Cu—Br]₃. Room temperature NMR data: ¹H NMR (in CDCl₃): δ (ppm) 1.00 (t, J=7.3 Hz, 6H, CH₃), 2.02 (br s, 4H, CH₂), 4.28 (br s, 1H, CH₂), 4.47 (br s, 1H, CH₂), 5.37 (br s, 1H, CH). ¹⁹F NMR (CDCl₃): δ (ppm) −60.06 (s). ¹³C{¹H} NMR (CDCl₃): δ (ppm) 13.5 (s, CH₃), 27.1 (s, CH₂), 81.3 (br s, ═CH₂, no=CHC₂H₅ peak was observed), 91.0 (s, C-4), 120.7 (q, ¹J_(C-F)=270.3 Hz, CF₃), 140.9 (q, ²J_(C-F)=35.6 Hz, C-3/C-5). Free 1-butene and [Cu—Br]₃ generated as a result of dissociation of 1-butene from [Cu—Br.(C₄H₈)] also present in the mixture and their signal were also observed. All these species are in a dynamic equilibrium. Low temperature (−60° C.) NMR data: ¹H NMR (in CDCl₃): δ (ppm) 0.97 (t, J=7.3 Hz, 3H, CH₃), 1.91 (br d, 2H, CH₂), 4.15 (d, 0.5H, CH₂), 4.25 (d, 0.5H, CH₂), 4.34 (d, 1H, CH₂), 5.28 (br d, 1H, CH). ¹⁹F NMR (CDCl₃): δ (ppm) −59.54 to −59.87 (several fine singlets). ¹³C{¹H} NMR (CDCl₃): δ (ppm) 14.2 (s, CH₃), 14.6 (s, CH₃), 27.0 (s, CH₂), 27.2 (s, CH₂), 80.2 (s, ═CH₂), 80.9 (s, ═CH₂), 90.8 (s, C-4), 107.7 (s, ═CHC₂H₅), 107.9 (s, ═CHC₂H₅), 120.3 (q, ¹J_(C-F)=268.3 Hz, CF₃), 140.2 (br q, C-3/C-5). Very minor peaks for free 1-butene and [Cu—Br]₃ were observed in ¹H and ¹⁹F NMR, however no such peaks were observed in ¹³C{¹H} NMR.

Characterization

[Cu—H]₃ and [Cu—Br]₃ react with 1-butene in solvents like dichloromethane to yield [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂, respectively (FIG. 7 ). The alkene products are obtained as colorless crystalline solids upon cooling the solutions under a 1-butene atmosphere. These [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ complexes quickly lose 1-butene under reduced pressure, and more slowly upon exposure to air, reverting to the precursor pyrazolates [Cu—H]₃ and [Cu—Br]₃ as evident from the spectroscopic and elemental analysis data of vacuum dried materials. The 1-butene loss is somewhat faster in [Cu—H.(C₄H₈)]₂.

The [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ complexes were isolated for spectroscopic studies without significant loss of 1-butene by drying crystalline samples using a gentle stream of 1-butene. Raman data of the solid samples indicate the presence of signals attributable to copper bound 1-butene (C═C stretching bands at 1534 and 1535 cm⁻¹) for [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂, respectively. These values represent a significant reduction (˜108 cm⁻¹) in C═C stretch energy relative to that of the free 1-butene (1643 cm⁻¹), as expected from the σ- and π-interactions of olefin with the copper(I). Related propene and ethene complexes of copper, [Cu—H.(C₃H₆)]₂ and [Cu—H.(C₂H₄)]₂ show 110 and 86 cm⁻¹ reduction in C═C stretching frequency upon coordination, relative to the corresponding free olefins.

Crystals of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ were investigated using X-ray crystallography at 100 K (FIG. 8 ). In contrast to the trinuclear and planar precursors [Cu—H]₃ and [Cu—Br]₃, 1-butene complexes are dinuclear species with six-membered Cu₂N₄ cores that adopt boat conformations. The copper centers have a trigonal planar geometry. The two η²-bound, 1-butene moieties display trans-orientation (FIG. 8 ), although both trans- and cis-orientation of the olefinic moieties are observed in the related propene complex in the solid state, which may be due to accommodating the sterics of the larger 1-butene moieties.

Selected bond distances and angles of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂, as well as of the analogous propene and ethene complexes are summarized in Table 1. There are no structurally characterized 1-butene complexes of copper for comparison.

TABLE 1 Selected bond distances (Å) and angles (deg) of copper(I) 1-butene complexes, [Cu—H•(C₄H₈)]₂ and [Cu—Br•(C₄H₈)]₂ Compound C═C Cu—C(H₂) Cu—C(H)Et C—Cu—C Cu—C—C—Et [Cu—H•(C₄H₈)]₂ 1.382(12) 2.004(8) 2.036(8) 40.0(3) 101.4 trans-orientation 1.364(12) 2.014(8) 2.016(8) 39.6(3) 100.7 (two molecules in the 1.373(13) 2.010(8) 2.040(7) 39.6(4) 102.3 asymmetric unit) 1.371(12) 2.018(8) 2.036(8) 39.5(3) 103.0 Av. 1.373 Av. 2.011 Av. 2.032 Av. 39.4 [Cu—Br•(C₄H₈)]₂ 1.368(5) 2.031(3) 2.047(3) 39.21(14) 100.6 trans-orientation 1.369(5) 2.019(3) 2.061(3) 39.19(14) 104.8 Av. 1.368 Av. 2.025 Av. 2.054 Av. 39.2

The average C═C distances of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ (1.373 and 1.368 Å, respectively) are slightly longer compared to 1.338 Å in free 1-butene. For comparison, the corresponding distance in the 1-butene complex of (η⁵-Cp)₂Zr(PMe₃)(C₄H₈) is relatively longer at 1.47(1) and 1.42(1) Å (for two independent molecules in the asymmetric unit), while those of (t-Bu₃SiO)₃Ta(C₄H₈) and [Ph(Me)CHNH₂]PtCl₂(C₄H₈) at 1.395(7), and 1.350(3) Å, respectively are not statistically different considering the esd values. These bond distance values point to a relatively stronger metal-butene bonding interaction in the Zr(II) complex, compared to [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂. The degree of bending of the ethyl substituent out of the olefinic plane (as evident from the Cu—C═C-Et torsion angle) also provides clues to the magnitude of σ/π-bonding interaction between Cu(I) and olefin of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂, and they are ay. 101.9° and 102.7° for the two complexes. They show much smaller bending of the ethyl substituent out of the olefinic plane, relatively to 1-butene complexes of Zr(II), Ta(III), and Pt(II) (their M-C═C-Et torsion angles are 125.2°, 121.7°, and 109.6°, respectively), and follow the observations noted for a larger data pool involving styrene and metal ions.

A comparison of metrical parameters of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ to their ethene and propene counterparts (Table 1) show that the C═C bond lengths and Cu—C distances are very similar for these copper olefin complexes. For example, average C═C distances vary from 1.364-1.374 Å, suggesting that the alkyl chain folds away and the sterics do not affect alkene coordination in these systems. All these copper(I)-olefin complexes feature boat-shaped Cu₂N₄ cores, but even planar conformation was observed in a copper(I) carbonyl complex [Cu—H.(CO)]₂ (which however involves, different, head-on bound CO than side-on bound olefin ligands).

[Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ was investigated as well as the corresponding propene and ethene complexes computationally using dispersion corrected DFT including both trans- and cis-isomers of 1-butene and propene complexes in terms of olefin orientation. Available experimental data agree with the metrical parameters of the optimized structures. For 1-butene complexes, the trans-species is favoured by 0.93 and 1.41 kcal·mol⁻¹ (3.9 and 5.9 kJ·mol⁻¹) for [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂, respectively. For the propene species [Cu—H.(C₃H₆)]₂ in which both cis- and trans-conformations were observed in crystalline products, the two isomers are energetically degenerate, while for [Cu—Br.(C₃H₆)]₂, the cis-isomer is favoured by a small margin, 0.32 kcal·mol⁻¹.

The calculated Raman ν(CH₂═CH₂), ν(H₂C═CHCH₃), and ν(H₂C═CHC₂H₅) for the most favoured isomer exhibit values of 1524, 1525, 1517 cm⁻¹ respectively, for the [Cu—H] systems, and 1533, 1535 and 1519 cm⁻¹ for the [Cu—Br] counterparts. Such values are in line with the experimental data, denoting a slight weakening of the Cu-alkene interaction for the brominated species. For comparison, computed ν(C═C) for free ethene, propene, and 1-butene are 1634, 1649, and 1642 cm⁻¹, respectively. Note that in ethylene complexes, it is important to consider ν(C═C) in conjunction with other pieces of evidence such as metric, theoretical, NMR spectroscopic for the analysis of metal-ethene bonding as done in this manuscript because the ν(C═C) stretch may couple with δ(CH₂) modes.

To probe the trimer to dimer equilibrium in more detail, ¹H, ¹³C, and ¹⁹F NMR were performed in CDCl₃ solution. The dimeric copper 1-butene complexes [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ exist in equilibrium with the trimeric [Cu—H]₃ and [Cu—Br]₃ precursors. For example, the room temperature ¹H NMR spectrum of [Cu—H.(C₄H₈)]₂ in CDCl₃ exhibited two peaks at δ 6.81 and δ 7.02 ppm that can be assigned to the protons on the pyrazolyl rings of [Cu—H.(C₄H₈)]₂ and the precursor [Cu—H]₃, respectively. Likewise, signals attributable to bound and free olefins are present but are broad, suggesting the existence of a dynamic equilibrium. ¹⁹F NMR spectrum shows two singlets, one at δ-60.12 and the other at δ−61.04 ppm for the two adducts (peak integration values of ¹H and ¹⁹F signals point to ˜1:1 molar ratio of the two species at 20° C.). Addition of excess 1-butene to this mixture leads to the formation of more [Cu—H.(C₄H₈)]₂ from [Cu—H]₃ as evident from the enhancement and diminution of NMR signals corresponding to the former and latter, respectively. Cooling the solution [Cu—H.(C₄H₈)]₂ biases the equilibrium toward [Cu—H.(C₄H₈)]₂, suggesting that the enthalpy change for the binding of 1-butene is exothermic in solution. Van′ t Hoff analysis of the VT-NMR data provided the enthalpy change for alkene uptake in solution as −34 kJ per mole of Cu-butene interaction for the formation of [Cu—H.(C₄H₈)]₂. The computed values for this process are in good agreement with the experimental observations (˜33.5 and −34.5 kJ·mol⁻¹ Cu for the cis- and trans-isomer formation, respectively), giving further credence to the computational results and insights generated (Table 2).

TABLE 2 Experimental and calculated enthalpy change (—H) given in kJ · mol⁻¹, per trimer and per Cu-atom. ΔH_(alkene/Cu3) ΔH_(alkene/Cu) Complexes (in solution, kJ · mol⁻¹) (in solution, kJ · mol⁻¹) [(3,5-(CF₃)₂Pz)Cu(H₂C═CH₂)]₂ −104 ± 3 −35 ± 1 [Cu—H•(C₂H₄)]₂ (calc. −124.6) [(4-Br-3,5-(CF₃)₂Pz)Cu(H₂C═CH₂)]₂  −67 ± 5 −22 ± 2 [Cu—Br•(C₂H₄)]₂ (calc. −72.8) [(3,5-(CF₃)₂Pz)Cu(H₂C═CHCH₃)]₂ −118 ± 6 −39 ± 2 [Cu—H•(C₃H₆)]₂ (calc. −132.7 cis) (calc. −132.7 trans) [(4-Br-3,5-(CF₃)₂Pz)Cu(H₂C═CHCH₃)]₂ −101 ± 6 −34 ± 2 [Cu—Br•(C₃H₆)]₂ (calc. 109.2 cis) (calc. −108.9 trans) [(3,5-(CF₃)₂Pz)Cu(H₂C═CHC₂H₅)]₂ −103 ± 6 −34 ± 2 [Cu—H•(C₄H₈)]₂ (calc. −100.4 cis) (calc. −103.6 trans) [(4-Br-3,5-(CF₃)₂Pz)Cu(H₂C═CHC₂H₅)]₂ −92 ± 5 −31 ± 2 [Cu—Br•(C₄H₈)]₂ (calc. −99.1 cis) (calc. −105.0 trans)

Computed Gibbs free energy (ΔG) based on experimental enthalpy and entropy values (Table 2), indicates that the adduct formation is exothermic below 258 K for [Cu—H.(C₂H₄)]₂, which increases to 327 K for [Cu—Br.(C₂H₄)]₂, in line with the variations observed for respective enthalpy changes (FIG. 9 ). For [Cu—H.(C₃H₆)]₂ and [Cu—H.(C₄H₈)]₂, ΔG for olefin complex formation is exothermic below 248 K for both adducts, while it increases to 275 K and 288 K, respectively for the corresponding [Cu—Br] derivatives.

Although only trans-[Cu—H.(C₄H₈)]₂ is observed in the solid-state, the ¹⁹F NMR spectrum at −40° C. shows four peaks indicating the existence of both the cis and trans isomers in solution. The ¹³C{¹H} NMR spectrum also shows two resonances each for the copper bound CH₂=(δ 79.4 and 80.0 ppm) and ═CH(Et) (δ 106.3 and 106.6 ppm) carbons, consistent with this conclusion. This interesting feature suggests that the steric bulk of the butene approaches a balancing point where both the cis- and trans-isomers are stabilized in solution, but (in contrast to the propene analogues) causes too much steric strain to favour the trans-isomer in the solid-state.

These ¹³C NMR chemical shift values represent a coordination induced upfield shift (Δδ=δ(free)−δ(coordinated)) of the CH₂═ and ═CH(Et) resonance by about 33.5 and 34.2 ppm respectively, relative to the corresponding peaks of the free 1-butene (δ 113.2 and 140.7 ppm for methylene and methyne carbon atoms, respectively). Copper complexes of 1-butene are remarkably rare. The ¹³C NMR data of [{bis(2-pyridyl)amine}Cu(1-butene)][BF₄] are available for comparison, and show that its CH₂═ and ═CH(Et) resonances appear very similar, δ 80.7 and 107.6 ppm, respectively. The 1-butene complex of Fe(II), [(η⁵-Cp)(CO)₂Fe(C₄H₈)][PF₆] in contrast, displays its CH₂═ and ═CH(Et) carbon resonance at a significantly higher upfield region (δ 54.5 and 90.9 ppm, respectively) suggesting a stronger Fe(II)-butene interaction compared to that in [Cu—H.(C₄-C₈)]₂. Both the Raman data and the olefinic carbon upfield shift of [Cu—H.(C₄-C₈)]₂ are in good agreement with the data on other types of olefin-copper complexes (e.g., copper(I) ethene or styrene) in the literatures.

The [Cu—Br.(C₄H₈)]₂ complex shows similar NMR data and enthalpy change value as noted above for the non-brominated, [Cu—H.(C₄-C₈)]₂ (Table 2). Overall, a comparison of ¹³C NMR data of butene, propene and ethene complexes of copper(I) systems, [Cu—H] and [Cu—Br], to literature data of d-block olefin complexes suggest that these copper-complexes, provided that there are no other complicating factors such as charges, display stronger σ/π-bonding interactions than Ag(I) but weaker than systems involving Au(I), Fe(II) noted above, Ni(0), and Ta(III).

To gain a deeper understanding of the nature of copper-olefin interaction in [Cu—H.(olefin)]₂, the contributions of different types were evaluated via the Morokuma-Ziegler energy decomposition approach (Table 3), which indicated a value of −42.4 kcal·mol⁻¹ for [Cu—H.(C₂H₄)]₂, −44.7 kcal·mol⁻¹ for [Cu—H.(C₃H₆)]₂, and −43.6 kcal·mol⁻¹ for [Cu—H.(C₄-C₈)]₂. In this framework, the interaction energy (ΔE_(int)) shows a more electrostatic character (60%) for [Cu—H.(C₄-C₈)]₂ species, while the orbital character of the interaction accounts for ˜33% of the stabilization involving both σ-donation and π-backbonding Cu-olefin bonding patterns (FIG. 10 ), which contributes similarly to the bonding scheme denoted by ΔE_(orb) stabilizing term (approximately ˜41% and ˜36%, respectively to ΔE_(orb)). Smaller alkenes, ethene and propene also show the same pattern of interaction with copper(I). For related [Cu—Br] species, similar values are obtained for the Cu-olefin interactions. However, changes are observed for Cu-pyrazole interaction, where for [Cu—Br] species, a decrease in energy is given by the smaller electrostatic character of the interaction, in contrast to the orbital and dispersion contribution which remains almost invariant when going from [Cu—H] to [Cu—Br] systems.

Preliminary calculations on isostructural, [3,5-(CF₃)₂Pz]⁻ ligand supported Ag(I), Fe(II), and Ni(0) olefin complexes related to the copper adducts investigated in this work exhibit a lowering to −26.8 kcal·mol⁻¹ of the olefin-metal interaction energy for [Ag—H.(C₄H₈)]₂, while the Fe(II) and Ni(0) complexes {[Fe—H.(C₄H₈)]₂}²⁺ and {[Ni—H.(C₄H₈)]₂}²⁻ show much larger values of −68.2 and −84.3 kcal·mol⁻¹, respectively. These interaction energies point to stronger σ-donor/π-backbonding capabilities of Cu(I) relative to Ag(I), among coinage metals, but not as high as those observed for Fe(II) and Ni(0) in comparable systems.

TABLE 3 Energy decomposition analysis of the interaction energy accounting for the complexation of one alkene group. Values in kcal · mol⁻¹. The most favourable isomer was considered. [Cu—H•(C₂H₄)]₂ [Cu—H•(C₃H₆)]₂ [Cu—H•(C₄H₈)]₂ ΔE_(Pauli) 113.9 125.9 121.7 ΔE_(elstat) −92.4 59.2% −100.6 59.0% −98.6 59.6% ΔE_(orb) −55.8 35.7% −58.1 34.0% −55.4 33.5% ΔE_(disp) −8.0  5.1% −11.8  6.9% −11.4  6.9% ΔE_(int) −42.4 −44.7 −43.6 Δρσ_(→Cu) ^(a) −21.8 39.0% −22.0 37.8% −22.8 41.1% Δρπ_(←Cu) ^(b) −20.3 36.3% −19.6 33.8% −19.9 36.0% ^(a)Accounts for the orbital contribution from σ-donation (Δρσ_(→Cu)) and ^(b)accounts for the orbital contribution from π-backbonding (Δρπ_(←Cu))

Initial measurements were performed by dosing [Cu—Br]₃ and [Cu—H]₃ with 1 atm of 1-butene and measuring the pressure drop over a two-hour period (FIG. 11 ). These experiments showed loadings of 0.15 and 0.27 mol_(butene)/mol_(complex), which are comparable to loadings of ethene and propene under similar experimental conditions. Increasing the feed pressure of 1-butene led to loadings of 2.5 and 0.82 mol_(butene)/mol_(complex) for [Cu—Br]₃ and [Cu—H]₃, respectively. This suggests that, despite its larger size, 1-butene is also able to induce the reversible solid-state to solid-state transformation observed for ethene and propene.

The rates of 1-butene, propene, and ethene adsorption into [Cu—Br]₃ and [Cu—H]₃ are shown in Tables 4 and 5, respectively. Despite the similar or larger kinetic diameter and critical volume of 1-butene (4.46 Å and 240.80 cm³/mol) compared to propene (4.5 Å, 184.6 cm³/mol) and ethene (3.9 Å, 131.1 cm³/mol), 1-butene adsorbed faster into [Cu—Br]₃. In contrast, for [Cu—H]₃ the rate of 1-butene adsorption was slower than propene and ethene. It is difficult to pin-point this slower 1-butene uptake in [Cu—H]₃ to a single clear-cut reason. The gas absorption chemistry in these non-porous [Cu—H]₃ and [Cu—Br]₃ solids is a complex process, which is accompanied by a separation of trimeric moieties to accommodate 1-butene, large structure-rearrangement involving multiple Cu—N bond breakages and formations, and Cu-olefin bond formations. Furthermore, [Cu—H]₃ exists as supramolecular-columns with long inter-trimer Cu . . . Cu interactions while [Cu—Br]₃ has a ladder structure with inter-trimer Cu . . . Br contacts.^([28, 41])

TABLE 4 The 1-butene, propene, and ethene uptake rates of [Cu—Br]₃ measured using the pressure drop method. The adsorption rate is divided into three regions, 0-1 min; 1-40 min; 40-120 min Average rate (0-1 min), Average rate (1-40 min), Average rate (40-120 min), Gas (mol_(ethene)/mol_(complex)/min) (mol_(ethene)/mol_(complex)/min) (mol_(ethene)/mol_(complex)/min) 100 kPa 1-Butene 0.082 ± 0.004  0.0074 ± 0.0001  0.0011 ± 0.00003 Propene 0.023 ± 0.002 0.0087 ± 0.001 0.0017 ± 0.0006 Ethene 0.021 ± 0.001 0.0075 ± 0.001 0.0019 ± 0.0005 180 kPa 1-Butene  1.5 ± 0.04 0.018 ± 0   0.001 ± 0    520 kPa Propene 0.82 ± 0.1  0.0011 ± 0.002 0.0010 ± 0.0005 600 kPa Ethene  1.2 ± 0.01 0.0079 ± 0.001 0.0005 ± 0.0001

TABLE 5 The 1-butene, propene, and ethene uptake rates of [Cu—H]₃ measured using the pressure drop method. The adsorption rate is divided into three regions, 0-1 min; 1-40 min; 40-120 min Average rate (0-1 min), Average rate (1-40 min), Average rate (40-120 min), Gas (mol_(ethene)/mol_(complex)/min) (mol_(ethene)/mol_(complex)/min) (mol_(ethene)/mol_(complex)/min) 100 kPa 1-Butene 0.023 ± 0.005 0.0089 ± 0.001  0.0008 ± 0.0002 Propene N/A N/A N/A Ethene N/A N/A N/A 180 kPa 1-Butene 0.59 ± 0.01 0.004 ± 0.0003 0.0004 ± 0.0002 520 kPa Propene 0.82 ± 0.2  0.04 ± 0.003 0.0015 ± 0.0006 600 kPa Ethene  1.2 ± 0.09 0.02 ± 0.003  0.003 ± 0.0007

Quantitative adsorption kinetics are rarely reported in the literature, mainly qualitatively accessed as ‘fast’ or ‘slow’ or inferred from breakthrough studies. Data for Zeolite 4A indicated that the porous adsorbent approximated equilibrium capacity within 15 minutes, slightly faster (though not dramatically) than the non-porous adsorbents in this study. This suggests that perceptions of adsorption rate being a fundamental limitation for non-porous adsorbents is based on intuition rather than data.

Retaining operating capacity over multiple cycles is key to adsorbent performance in an industrial process. The non-porous adsorbents [Cu—Br]₃ and [Cu—H]₃ retained capacity over 5 cycles of ethene, propene, and 1-butene at 1 bar and the higher pressures as indicated in Tables 4 and 5. This extensive cycling (up to 30 cycles without degradation) supports the potential application of these complexes to gas separation processes.

Desorption is another vital parameter to evaluate for potential applications to gas separations processes. Samples of [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ were qualitatively observed to slowly lose 1-butene when exposed to atmosphere, similar to the ethene and propene complexes reported previously. Because of the relatively slow rate of butene loss, thermogravimetric analysis (TGA) was used to evaluate the thermal desorption conditions required to regenerate the 1-butene complexes of [Cu—Br]₃ and [Cu—H]₃. The [Cu—Br]₃, previously also reported to lose ethene and propene slowly, retained some 1-butene up to ca. 175° C. then began to decompose. In contrast, [Cu—H]₃ had already lost ca. half its 1-butene loading by the time it was unloaded from the high-pressure cell (ca. 5 minutes) and completed desorption by ca. 50° C., which may be more related to the time of exposure to atmosphere than heating. [Cu—H]₃ and [Cu—Br]₃ showed similar decomposition temperatures (175° C.). The relatively large size of 1-butene and its ability to rapidly desorb through dense [Cu—H]₃ raises further intriguing questions about the mechanism of adsorption and desorption of gas molecules through these dense crystalline materials.

Positron Annihilation Spectroscopy (PALS) was performed to provide insight into the differences in rate of adsorption and desorption for [Cu—Br]₃ and [Cu—H]₃ (Table 6). PALS is an emerging characterization technique which uses positrons to probe the free-volume elements within materials. Positrons are attracted to areas of low electron density and will annihilate when interacting with matter, therefore the lifetime (T3) is proportional to the size of the free volume elements (Diameter 3) within the material. The associated Intensity (I₃) is related to the relative number of free volume elements. The average free volume element sizes within [Cu—Br]₃, and [Cu—H]₃ were 0.581, and 0.301 nm respectively. [Cu—Br]₃, had the larger average free volume element size, allowing for the high uptakes at ≤100 kPa. The sample, however, caused positron inhibition as was evidenced from the very low intensity <1%. Therefore, 1₃ is not representative of the number of free volume elements within the sample. Although [Cu—H]₃ had the shorter lifetime, and hence smaller free volume element size, it featured high Intensity, therefore showing considerable number of accessible free volume elements. The smaller free volume element size, 0.3 nm, is too small for the adsorption of ethene or butene through the solid which would explain why increased pressure is needed to convert the structure. The size difference could also account for the faster rate of uptake for [Cu—Br]₃ compared to [Cu—H]₃. The kinetics of desorption were too fast to observe the structural changes from ethene treatment and needs high pressure analysis.

TABLE 6 PALS results for solid [Cu—Br]₃ and [Cu—H]₃ under dynamic vacuum. Free Volume Element Sizes Lifetimes Intensity Diameter Sample Treatment τ₃ (ns) ± I₃ (%) ± 3 (nm) ± [Cu—Br]₃ Vacuum 2.057 0.195 0.6 0.1 0.581 0.035 [Cu—H]₃ Vacuum 0.911 0.005 34.1 1.3 0.301 0.002

1-Butene was reacted with [Cu—Br]₃ and [Cu—H]₃ in both the solution and solid states. In solution, the trimeric species rearrange to the dimeric complexes [Cu—Br.(C₄H₈)]₂ and ([Cu—H.(C₄H₈)]₂. NMR, Raman, X-ray, and computational studies were used to examine the nature of the copper(I)-alkene interactions and compare the bonding, structural and spectroscopic features in the ethene, propene, butene series. For the first time, copper(I) complexes of 1-butene were characterized using single crystal X-ray crystallography. Isolable [Cu—H.(C₄H₈)]₂ and [Cu—Br.(C₄H₈)]₂ would serve as useful models for species that may be present in copper containing porous materials or solutions^([18, 20]) utilized for 1-butene/butane separation. The adsorption of gaseous 1-butene by solid [Cu—Br]₃ and [Cu—H]₃ also has remarkable features. The significantly larger 1-butene is somehow able to penetrate the dense solid material and to coordinate with copper(I) ions at high molar occupancy. The adsorption of 1-butene into these non-porous adsorbents occurs over similar timescales to porous adsorbents, removing one roadblock towards application in gas separations. 

What is claimed is:
 1. A composition comprising an alkene and a compound having Formula I:

wherein each X is, independent of the other, chosen from H and Br; and each Y is, independent of the other, chose from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I.
 2. The composition of claim 1, wherein the alkene is ethene, propene, 1-butene, or a mixture thereof.
 3. The composition of claim 1, wherein Y is CF₃.
 4. The composition of claim 1, further comprising an alkane.
 5. The composition of claim 1, further comprising a complex having Formula II:

wherein each X is, independent of the other, chosen from H and Br; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I.
 6. The composition of claim 5, wherein Alkene is ethene, propene, or 1-butene.
 7. A complex having Formula II:

wherein each X is, independent of the other, chosen from H and Br; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I.
 8. The complex of claim 7, wherein Alkene is ethene, propene, or 1-butene.
 9. The complex of claim 7, wherein Y is CF₃.
 10. A method of separating an alkene from a mixture comprising the alkene and an alkane, comprising: contacting the mixture with a compound having Formula I:

to form a complex having Formula II

wherein each X is, independent of the other, chosen from H and Br; and each Y is, independent of the other, chosen from CF₃, C₂F₅, C₃F₇, C₄F₈, F, Cl, Br, and I.
 11. The method of claim 10, wherein Y is CF₃.
 12. The method of claim 10, wherein the alkene is ethene, propene, 1-butene, or a mixture thereof.
 13. The method of claim 10, wherein the alkane is ethane, propane, butane, or a mixture thereof.
 14. The method of claim 10, wherein the mixture is contacted with the compound having Formula I at a pressure below a partial pressure of the alkene.
 15. The method of claim 10, wherein the mixture is contacted with the compound having Formula I at a temperature from 0° C. to 200° C.
 16. The method of claim 10, wherein the mixture is contacted with the compound having Formula I at a pressure from ambient pressure to 100 kPa.
 17. The method of claim 10, wherein the mixture is contacted with the compound having Formula I at a pressure from 100 kPa to 100,000 kPa.
 18. The method of claim 10, wherein the mixture is contacted with the compound having Formula I at a pressure from 600 kPa to 1000 kPa.
 19. The method of claim 10, further comprising reducing pressure to ambient pressure or below after forming the complex having Formula II, and collecting the alkene.
 20. The method of claim 10, further comprising increasing temperature after forming the complex having Formula II, and collecting the alkene.
 21. The method of claim 10, wherein the mixture is contacted with the compound having Formula I in the presence of a solvent. 